FLURRIES OR FEET? An Analysis of QPF Errors in an Eastern North Carolina Snowstorm December 2-4, 2000 Jason Caldwell South Carolina State Climate Office.

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Presentation on theme: "FLURRIES OR FEET? An Analysis of QPF Errors in an Eastern North Carolina Snowstorm December 2-4, 2000 Jason Caldwell South Carolina State Climate Office."— Presentation transcript:

1 FLURRIES OR FEET? An Analysis of QPF Errors in an Eastern North Carolina Snowstorm December 2-4, Jason Caldwell South Carolina State Climate Office

6 What we know:Model forecasts missed a precursor vorticity maximum at 500 mbModels predicted coastal front too close to coast (site for surface cyclone formation)Models under-predicted intensity of cold-air dammingModels produced heavy precipitation with a thermodynamic profile that was characterized by shallow cloudOperational sea-surface temperature analyses were too warm according to NCEP, could have been tied to coastal front errors

12 HOW DOES THIS AFFECT THE QPF?The strength and southern extent of the cold air damming was under-predicted by model forecasts and led to errors in QPF.HOW DOES THIS AFFECT THE QPF?The decreased magnitude of CAD shifts the location of the coastal front and affects low-level thermal and moisture profiles inland- thermal gradient (baroclinic zone) is positioned farther west due to restricted areal coverage of cold dome- moisture availability is higher due to less airmass intrusion from parent high- lower atmosphere is less stable and more conducive to vertical motion- coastal front more likely to propagate inland as WAA overtakes weaker CAD

13 HOW DOES THIS AFFECT THE QPF?Model forecasts predicted the placement of the coastal front west of the actual location off the coast of North Carolina and resulted in QPF errorsHOW DOES THIS AFFECT THE QPF?Low pressure forms closer to the coast which extends the western boundary of precipitation into the Triad region- enhanced upward vertical motion inland due to surface convergence at CF- increased moisture transport with stronger and more easterly winds at low-levels- prolonged duration of precipitation as a result of proximity- prolific isentropic lift as warm air advection associated with cyclogenesis moves into NC over CAD at mid-levels

14 HYPOTHESIS BModel microphysics were overactive in producing precipitation with dry air aloft and a shallow cloudEVALUATE PRECIPITATION MECHANISMS PRESENTDIAGNOSE VERTICAL TEMPERATURE/MOISTURE PROFILE OVER CENTRAL NCHOW DOES THIS AFFECT QPF?

18 HOW DOES THIS AFFECT THE QPF?The ETA model microphysics scheme allowed ice crystal growth at –10 degrees C which allowed precipitation generation to occur in shallow, super-cooled water droplet clouds.HOW DOES THIS AFFECT THE QPF?Dendrite growth occurred in an non-conducive thermal environment generating spurious precipitationIncreased moisture availability in the models through an extended depth over-quantified precipitation totals, when in reality the atmosphere was dry above 700mb